A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can for example be used when manufacturing integrated circuits (ICs). In general, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) of a substrate (e.g. a silicon wafer). The transfer of the pattern is typically carried out by imaging it onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The radiation used to expose the target portion with a pattern is projected by a projection system. In order to obtain small resolutions, high numerical apertures (NA) are applied in such projection systems. However, in such a lithographic apparatus, frequently sensors are used that measure different parameters under such high NA conditions. A first example is a sensor that measures the aberrations of the projection system, such as described in U.S. published application 2002-0001088 A1. A second example is a transmission image sensor that measures the position of an image of the pattern formed relative to the substrate, such as described in EP1 510 870 A 1.
Such sensors, described in U.S. published application 2002-0001088 A1 and EP 1 510 870 A1, have to be able to effectively measure high NA radiation impinging on the sensor. When the sensors are not designed for use with high NA radiation, radiation will be lost and less accurate measurements may be performed. Loss of radiation may occur because of scattering at a rough surface within the sensor. It may also occur because of reflection at an optically smooth interface with a refractive index mismatch, i.e. an interface between two materials with smooth surfaces, but having a large difference in refractive index. A third cause of loss of radiation is total internal reflection at the interface between a material with a high refractive index and a low refractive index. This may cause radiation to leave the layer with the high refractive index on a side perpendicular to the interface after reflection of the radiation towards the side. With the radiation intended to cross the interface, radiation leaving the layer with the high refractive index on the side is lost. A fourth cause of loss of radiation is absorption in air gaps within the sensor. The air gaps cannot easily be purged and therefore may contain air and water, which absorb radiation of 157 nm wavelength. This is a typical wavelength for irradiating a target portion with a pattern.
EP 1 510 870 A1 discloses a sensor for use in a lithographic apparatus arranged to avoid loss of radiation within the sensor. It proposes to use filler layers in the sensor to avoid these problems. It, for instance, suggests using liquids such as Fomblin to form the filler layers. However, EP 1 510 870 A1 does not disclose how the liquids can be contained in the sensor. It is necessary to make sure that the liquid is kept at its intended position because the sensor is typically mounted on a substrate support table that moves at high speed. For instance, the substrate support table is moved at high speed while stepping to bring a second target portion below the projection system, after the first target portion has been irradiated. Thus, if the liquid is not properly contained, it flows away from its intended position during such a movement.
At first, seals were considered for sealing the liquid inside the sensor thereby solving the above drawback. During production of a sensor, there are always tolerances. The height of the filler layer is subject to tolerances as well. Therefore, when designing parts to seal the liquid inside the sensor, these height tolerances must be accounted for. Therefore flexible seals were considered.
A problem with the use of flexible seals, such as those made from rubber, within a sensor as disclosed above in EP 1 510 870 A1, is that flexible seals comprise molecules that gas out. In a lithographic apparatus, the environmental conditions are controlled with high accuracy to obtain very small patterns consistently at high yield. Variations in the material between the projection lens and the substrate cause variations in the index of refraction experienced by the radiation beam. This will cause variations in the image of the circuit pattern formed on the layer of radiation-sensitive material. Molecules that have gassed out of the flexible seals cause such variations in the material. Moreover, a lithographic apparatus normally comprises many very sensitive parts, such as interferometers, optical level sensors and optical alignment sensors. On all these sensitive parts, the gassed out molecules could form sediments, influencing the performance of the parts. A lithographic apparatus is regularly placed in a foundry that also houses other very sensitive apparatus, which performance may be negatively influenced by gasses entering those other apparatus. Thus, the lithographic apparatus should fulfill extremely tight specifications to reduce influencing the environment, such as by releasing materials. Having these kinds of molecules gassing out of a flexible seal into a lithographic apparatus is therefore considered as highly undesirable contamination of the machine. Other drawbacks exist with known systems.